1. Field of the Invention
This invention relates to the field of flow control in a fluid. More specifically, the invention comprises the use of properly placed microjets to control flow separation and or recirculation over a given surface.
2. Description of the Related Art
Flow separation is defined as the detachment of a flowing fluid from a solid surface. It is generally caused by a severe pressure gradient. The gradient itself may result from a geometric feature on the solid surface, or simply placing the surface at a high angle of attack with respect to the airstream. Whatever the cause, flow separation produces a significant thickening of the turbulent region adjacent to the solid surface. The boundary layer may even detach from the surface to produce a region of reverse flow. Such reverse flow can be intermittent or continuous.
Flow separation is undesirable in many applications. One example is the complex inlet ducting used to feed air to an aircraft engine. Such inlets are now commonly curved, so that the high radar signature of the compressor will not be directly visible. FIG. 1 illustrates a serpentine inlet 10, which is one example of many types. The intake is toward the left side of the view with the engine compressor being located proximate the exhaust portion in the right side of the view. The reader will observe that the air flow bends through a circuitous path and transitions from a four-sided intake section to the round section needed at the compressor intake.
Such an inlet is designed to handle large amounts of air flow. Flow separation is a known problem in such applications. Those skilled in the art will know that the serpentine may experience variable flow. As the aircraft maneuvers—often undergoing substantial angles of attack in pitch and yaw—the pressure distribution across the intake varies significantly. This variance produces flow separation in different locations at different times. A substantial flow separation can degrade the engine performance and even lead to compressor stall.
The prior art includes several approaches to reducing and controlling flow separation. These include: (1) Injecting pressurized air in a direction which is tangential to the flow—such as slotted aircraft flaps; (2) Applying vacuum to the boundary layer by using vacuum orifices or a permeable surface; (3) Adding vortex generators, such as vanes or bumps; and (4) Adding forced excitation devices such as synthetic jets (which include no net mass flux, but create an effect similar to devices which add or subtract mass to the flow). The prior art approaches clearly indicate the desirability of controlling flow separation.
FIG. 3B provides an illustration of the prior art approach of injecting pressurized air in a direction which is tangential to the flow. Boundary layer separation occurs when flow momentum is lost and the air in the vicinity of the boundary layer stops moving and/or actually reverses direction. In the prior art, flow separation is sometimes reduced or eliminated using direct momentum injection. In FIG. 3B, direct momentum injector 80 injects a relatively high mass in the vicinity of high adverse pressure gradient 18. Added mass 82 possesses momentum “artificially” created by the injection process itself. Incoming boundary layer 42 is able to flow over the added mass and the prevailing flow thereby remains attached.
An example of this approach is disclosed in U.S. Pat. No. 5,447,283. The Tindall invention deals with an inlet for a jet aircraft engine. The aircraft in question is designed to operate in the subsonic, transonic, and supersonic regimes. Several different direct momentum injector nozzles are located at different points in the inlet. Each is fed by pressurized air taken from the jet engine's compressor (“bleed air”).
The amount of air injected in the Tindal device ranges between 1% and 3% of the total flow through the inlet. This is a relatively high flow rate. Ann exemplary military jet engine is the PRATT & WHITNEY F100 used in the F-15 and F-16 aircraft. A typical inlet flow rate for this engine is about 300 kg/second without the afterburner being lit. Using this exemplary flow rate, one may determine that the mass injected by the Tindall device varies between 3 kg/second and 9 kg/second. This is a very substantial mass injection rate.
As is known to those skilled in the art, the use of bleed air taken from an engine's compressor reduces the engine's performance. It is generally undesirable and the minimization of the amount of bleed air required is a well known goal. The prior art approach of direct momentum injection works in many cases, but it entails a significant performance penalty because of the mass flow rate required. It would therefore be preferable to provide a flow control device that uses a significantly lower mass flow rate. The present invention provides such a solution.